This invention relates to a method and apparatus for modifying a composite television signal to inhibit reproduction of an unauthorized recording thereof by conventional video recorders but enable the display of a video picture therefrom on a television receiver; and, more particularly, to a technique by which the vertical periods, i. e. the lengths of successive frames, of the television signal are varied under the control of a "profile pattern" which may be easily adjusted so as to correspondingly adjust the manner in which the vertical period changes.
With the abundance of video tape recorders (VTR's) now in use in many homes, it has become commonplace for users to record off-the-air television programs for subsequent, and often repeated, viewing. In addition, consumers have enthusiastically accepted pre-recorded video programming, typically, commercially successful motion pictures; and this has resulted in large libraries of pre-recorded video tapes for sale or rent to the public. While such legitimate recordings are welcomed, the financial profit associated with selling or renting pre-recorded video tapes has given rise to illegal piracy. So-called video tape pirates reproduce several, often hundreds of unauthorized copies of a video tape, thereby depriving the rightful owners or distributors of lawful income.
Television subscription networks, such as so-called sattelite or cable television distribution systems, face similar difficulties. To prevent adequate reception by non-subscribers, such television subscription networks typically encode, or "scramble", the distributed television signals, thereby defeating acceptable video displays of those television signals by non-subscribers who are not provided with proper decoders or "descramblers". However, a subscriber may simply connect a video recorder to his decoder so as to record for subsequent and repeated viewing a desired video program that is distributed over the subscription network. Such recording for later viewing decreases the market interested in a re-distribution of that video program over the subscription network. As this market decreases, individuals may terminate their subscriptions and the video program distributor (i. e. the cable network) may suffer financial damage.
Providers of subscription television programming have long proposed so-called "pay per view" broadcasting. This broadcasting contemplates a once-only distribution of valued video programming, such as first-run motion pictures, highly popular sporting events, special entertainment events, and the like, to subscribers who would be charged a one-time fee to receive that video program. Such one-time broadcasting is quite sensitive to video recording which, if permitted, would seriously erode the value of pay-per-view transmission.
Analogous to pay-per-view video distribution is the so-called "electronic theater". As presently envisaged, the electronic theater would be quite similar to a typical motion picture theater, except that actual prints or copies of a motion picture need not be used in each theater. Rather, high resolution television signals can be broadcast simultaneously to several theaters, such as by satellite transmission, for "real time" display to the theater audience. However, the success of the electronic theater may be contingent, in part, on the ability to prevent unauthorized recording and video tape duplication of the broadcast program.
Of course, scrambling or encoding of a video signal prevents a non-subscriber from recording the video program. However, an authorized subscriber or one who obtains a compatible descrambler may use his VTR to record the descrambled video program. It is preferred that the basic television signal be modified to the extent that even after scrambling/descrambling an acceptable video picture may be reproduced on a conventional television receiver, but the operation of a VTR should be defeated such that it cannot be used to record and reproduce a satisfactory video picture.
One technique proposed for making a television signal nonrecordable relies upon the automatic gain control (AGC) circuitry normally included in a VTR. A large pulse is inserted into the vertical blanking interval to "confuse" the AGC circuitry into substantially attenuating the video signal during recording, thereby making it quite difficult to reproduce a video signal of adequate level. It is believed that this proposal can be easily defeated and, thus, it does not adequately inhibit a VTR from recording and playing back the modified television signal. It also is believed that this technique will defeat the operation of certain addressable "descramblers" used in some cable systems, resulting in an unsatisfactory video picture ultimately displayed on a subscriber's television receiver.
Another technique for modifying a television signal to prevent its recording/reproduction relies upon the relative sensitivity of the vertical synchronizing signal detecting circuitry normally provided in virtually all VTR's. By removing a portion of the vertical synchronizing pulses included in the vertical blanking interval, the vertical synchronizing signal detector included in most VTR's will be unable to detect those vertical sync pulses, resulting in loss of critical servo control information needed for proper operation of the VTR. Since the vertical synchronizing circuitry included in most television receivers is not as sensitive, there is no loss of vertical synchronization in the television receiver. Recently, however, the vertical synchronizing signal detecting circuitry included in VTR's has been significantly improved, and in some instances digital techniques have been used, resulting in the ability of such VTR's to record and reproduce television signals that have been modified as aforesaid.
Various other proposals have been made regarding modification of the vertical synchronizing signal for the purpose of defeating the vertical sync locking circuitry normally provided in VTR's. Some proposals have suggested that some of the horizontal synchronizing signals be deleted from the transmitted television signal; but these suggestions are subject to the same difficulties associated with video signal scrambling techniques: one who has a descrambler or decoder may record the television signal.
One technique which may offer the promise of success contemplates a change in the length of the two video fields which constitute a frame of television signals. U. S. Pat. Nos. 4,488,176 and 4,673,981 both suggest that the frame length may be enlarged or reduced by adding or subtracting horizontal line intervals to each frame. Thus, the frame length varies from its nominal 33.33 milliseconds, depending upon the number of lines which have been added to or deleted from the video frame. In both proposals, however, the rate at which lines are added to and deleted from the frames is fixed, and over a period of time the number of lines which are added is equal to the number of lines which are deleted. Furthermore, in both proposals, the duration of each horizontal line interval is fixed at the standard 63.5 microsecond duration.